Summary:
Brucellosis was first detected in the Yellowstone Buffalo
herd in 1917. The buffalo were exposed to brucellosis by domestic
cattle that were grazed in the park and held in confinement
with buffalo. Brucellosis is most commonly transmitted among
and between species through ingestion of infected birthing
materials. Yellowstone buffalo developed a natural immune
response to brucellosis and do not typically suffer from the
disease. It is believed that many buffalo may also have a
genetic immunity to brucellosis. Failed pregnancies, the most
common symptom of brucellosis, are relatively unknown in Yellowstone
buffalo. The most likely mode of exposure among buffalo is
ingestion of small amounts of bacteria from newborn live calves.
Essentially, the buffalo in Yellowstone are vaccinating themselves
for brucellosis, developing an immune response, and clearing
the bacteria. There has never been a documented case of brucellosis
transmission between buffalo and domestic cattle under natural
conditions. In Grand Teton National Park, where vaccinated
cattle and brucellosis exposed buffalo have been commingling
for decades, no transmission has ever occurred. The chances
of transmission between wild buffalo and vaccinated domestic
cattle have been characterized as “very low”.

Issues: A number of factors including the
incidence and transmitability of brucellosis in buffalo, the
distribution of cattle in the Greater Yellowstone Area, and
the regulatory structure in place for brucellosis relate to
the current situation that has led to the death of nearly
4,000 buffalo since 1985.1. Testing methods: There are currently two
methods to test buffalo for brucellosis exposure and infection;
serology and culture. Serologic testing involves drawing blood
from live animals to determine if long-term antibodies for
brucellosis are present. Buffalo that test positive are considered
infected and sent to slaughter. Approximately 45 percent of
Yellowstone buffalo test sero-positive on the CARD test, the
one most commonly used by the agencies. Culture testing involves
tissue sampling from slaughtered buffalo to determine if actual
bacteria are present. Culture testing is considered to be
the “gold standard” in determining infection.
Drastic differences between sero-positive and culture positive
buffalo indicate that many buffalo are being slaughtered simply
because they have developed immunity to brucellosis and are
not actually infected. A combination of test results indicate
that only between 2 and 20 percent of buffalo actually have
brucellosis bacteria in their bodies at any given time.

2. Transmitability: The most likely method
of transmission between species is ingestion of infected birthing
materials from an aborted fetus. Buffalo bulls, calves, yearlings
and non-pregnant females do not pose a significant risk of
shedding infected materials in the environment. When a female
buffalo is infected with brucellosis, she will pass the bacteria
in her first pregnancy. After the first calving, the uterus
will “superprotect” itself from brucellosis preventing
infected material from being shed in subsequent calving even
if she is re-exposed. Therefore, only pregnant female buffalo
in the first calving cycle after exposure have the possibility
of shedding infected material in the environment. Brucellosis
related abortions, even among infected females, are extremely
rare in Yellowstone buffalo. Given the very small segment
of the population that can even potentially transmit brucellosis
combined with the low probability of transmission occurring
in natural settings, the real chances of brucellosis transmission
are extremely low. Additionally, brucellosis bacteria will
not survive in warm weather and direct exposure to sunlight,
and the activity of predators/scavengers all but guarantee
that fetuses or infected birthing material will not persist
in the environment beyond mid-May.

3. Distribution of Cattle: Relatively few
cattle graze in the GYA at any time of the year, particularly
in the winter and spring months when transmission is even
a possibility. In the Western Boundary Area, no cattle are
present within 45 miles of Yellowstone National Park in winter
and spring. Cattle are typically only in the area between
mid-June and mid-October, a period when there is no possibility
of brucellosis transmission. The vast majority of cattle that
graze in the summer in the Western Boundary Area are imported
from Idaho and are already subjected to brucellosis vaccination
and testing. In the Northern Boundary Area, there are never any cattle on the west side of the
Yellowstone River between Gardiner and Yankee Jim Canyon.
One producer grazes about 25 cows on the east side of the
Yellowstone River approximately 4 miles north of Gardiner
on Rt. 89. This same producer brings his cattle to private
land adjacent to the Eagle Creek Special Management Area in
the spring months. Untested buffalo are allowed to be in the
Eagle Creek SMA as well and no transmission of brucellosis
has ever occurred. One additional producer grazes cattle on
the east side of Yellowstone River north of Gardiner. Both of these ranchers have publically stated that they welcome wild bison on their properties and are not concerned about a brucellosis transmission.

4. Regulatory issues: Montana is currently
certified brucellosis class free. Class free status allows
producers to transport reproductive cattle across state lines
without brucellosis testing. The United States is not certified
brucellosis free by the OIE, the international regulatory
body. Therefore, brucellosis testing is required to transport
reproductive cattle across international boundaries. In order
for the US to be certified brucellosis free by the OIE, no
livestock in the country can have been vaccinated for three
years.

Conclusion: Only a relatively small percentage
of Yellowstone buffalo are actually infected with brucellosis.
Brucellosis does not have any significant impact on the health
of the Yellowstone buffalo. The risk of transmission from
wild buffalo to cattle is infinitesimally low. Relatively few susceptible
cattle graze in the GYA and most are not present when transmission
is even a possibility. Herd management plans that adjust stocking
dates could be developed to insure that transmission does
not occur. Montana can easily comply with the National Brucellosis
Eradication Program to insure that brucellosis class free
status is preserved. The GYA could be exempted from the OIE
certification process and allow the rest of the country to
enjoy international brucellosis free status. Montana can develop
risk management strategies for domestic cattle to allow for
wild, free roaming population of buffalo in GYA and beyond.